The purpose of this project is to develop methods for gene mapping in eukaryotic cells. Gene transfer is detected by isolation of colonies of biologically transformed cells in selective medium after incubation of isolated metaphase chromosomes with mutant recipient cells deficient in HPRT or TK. The gene product is further characterized as chromosomal donor species. Cotransfer of two closely linked genes, TK and galactokinase, occurs by this procedure. The method currently may be most useful for determining close gene linkages.